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Abstract:

A distortion compensation circuit and a distortion compensation method in
which signal level of a signal after distortion compensation is limited,
and which can improve the distortion compensation effect are provided. A
distortion compensation circuit of the present invention includes an
input level limitation means which limits signal level of an input signal
from outside to no more than the first limit value, and outputs a signal
before distortion compensation; a distortion compensation means which,
based on a distortion compensation coefficient, performs distortion
compensation processing to the signal before distortion compensation and
outputs a signal after distortion compensation; a distortion compensation
coefficient calculation means which, based on a difference between part
of output signals of an amplifier which amplified the signal after
distortion compensation and the signal before distortion compensation,
calculate a coefficient for compensating distortion of the output signal
of the amplifier as the distortion compensation coefficient; a storage
means which stores the distortion compensation coefficient by correlating
it to signal level of the signal before distortion compensation, and a
limit value calculation means which calculates signal level of the signal
before distortion compensation when signal level, when distortion
compensation processing is performed based on the distortion compensation
coefficient stored in the storage means, is no more than the second limit
value which is a limit value of signal level of the input signal to the
amplifier, as the first limit value.

Claims:

1. A distortion compensation circuit comprising:an input level limitation
unit that limits signal level of an input signal from outside to no more
than a first limit value, and outputs a signal before distortion
compensation;a distortion compensation unit that performs distortion
compensation processing to said signal before distortion compensation
based on a distortion compensation coefficient, and outputs a signal
after said distortion compensation;a distortion compensation coefficient
calculation unit that calculates a coefficient for compensating
distortion of an output signal of an amplifier as a distortion
compensation coefficient based on a difference between part of output
signals of said amplifier which amplified said signal after distortion
compensation and said signal before distortion compensation;a storage
unit that stores said distortion compensation coefficient by correlating
it to signal level of said signal before said distortion compensation,
anda limit value calculation unit that calculates signal level of said
signal before distortion compensation as said first limit value providing
the signal level, after said distortion compensation processing based on
said distortion compensation coefficient stored in said storage unit is
no more than a second limit value which is a limit value of signal level
of an input signal to said amplifier.

2. The distortion compensation circuit according to claim 1 wherein said
distortion compensation unit performs said distortion compensation
processing based on said distortion compensation coefficient
corresponding to signal level of said signal before distortion
compensation.

3. The distortion compensation circuit according to claim 2, whereinsaid
storage unit stores said distortion compensation coefficient by
correlating it to electric power of said signal before distortion
compensation, andsaid distortion compensation unit performs said
distortion compensation processing based on said distortion compensation
coefficient corresponding to electric power of said signal before
distortion compensation.

4. The distortion compensation circuit according to claim 2, whereinsaid
storage unit stores said distortion compensation coefficient by
correlating it to amplitude of said signal before distortion
compensation, andsaid distortion compensation unit performs distortion
compensation processing based on said distortion compensation coefficient
corresponding to amplitude of said signal before distortion compensation.

5. The distortion compensation circuit according to claim 1, whereinsaid
second limit value is a limit value which limits electric power of an
input signal to said amplifier, andsaid limit value calculation unit
calculates electric power of said signal before distortion compensation
as said first limit value providing the electric power after said
distortion compensation processing based on said distortion compensation
coefficient stored in said storage unit is no more than said second limit
value.

6. The distortion compensation circuit according to claim 1, whereinsaid
second limit value is a limit value which limits amplitude of an input
signal to said amplifier, andsaid limit value calculation unit calculates
amplitude of said signal before distortion compensation as said first
limit value providing the amplitude, after said distortion compensation
processing based on said distortion compensation coefficient stored in
said storage unit is no more than said second limit value.

7. The distortion compensation circuit according to claim 1, said input
level limitation unit comprising:a coefficient calculation unit that
calculates an input level limitation coefficient which limits signal
level of said signal before distortion compensation to be no more than
said first limit value by multiplying it to said input signal,
andmultiplying said input level limitation coefficient to said input
signal.

8. The distortion compensation circuit according to claim 7, wherein said
coefficient calculation unit judges whether to limit signal level of said
input signal by comparing signal level of said input signal and said
first limit value, and calculates said input level limitation coefficient
based on a result of said judgment.

9. The distortion compensation circuit according to claim 1, said input
level limitation unit comprising:a coefficient calculation unit that
generates a window function which limits signal level of said signal
before distortion compensation to be no more than said first limit value
by multiplying it to said input signal, andmultiplying said window
function to said input signal.

10. The distortion compensation circuit according to claim 9, wherein said
coefficient calculation unit judges whether to limit signal level of said
input signal by comparing signal level of said input signal and said
first limit value, and generates said window function based on a result
of said judgment.

11. The distortion compensation circuit according to claim 7, whereinsaid
first limit value is a limit value which limits electric power of said
input signal, andsaid coefficient calculation unit that compares electric
power of said input signal and said first limit value, and judges whether
to limit signal level of said input signal.

12. The distortion compensation circuit according to claim 7, whereinsaid
first limit value is a limit value which limits amplitude of said input
signal, andsaid coefficient calculation unit compares amplitude of said
input signal and said first limit value, and judges whether to limit
signal level of said input signal.

13. The distortion compensation circuit according to claim 1, wherein said
second limit value is saturated input signal level which makes an output
of said amplifier saturated.

14. The distortion compensation circuit according to claim 13, wherein
said second limit value is saturated input electric power which makes an
output of said amplifier saturated.

15. The distortion compensation circuit according to claim 13, wherein
said second limit value is saturated input amplitude which makes an
output of said amplifier saturated.

16. The distortion compensation circuit according to claim 13, wherein
said second limit value is a square root of saturated input electric
power which makes an output of said amplifier saturated.

17. The distortion compensation circuit according to claim 1, wherein said
distortion compensation coefficient calculation unit calculates said
distortion compensation coefficient so that a difference between part of
output signals of said amplifier and said signal before distortion
compensation becomes minimum.

18. The distortion compensation circuit according to claim 1, wherein said
distortion compensation coefficient includes an amplitude compensation
coefficient which compensates amplitude and a phase compensation
coefficient which. compensates phase of an output signal of said
amplifier.

19. A distortion compensation circuit comprising:a coefficient calculation
unit that judges whether to limit signal level of said transmission
signal by comparing signal level of a transmission signal inputted from
outside and a first limit value, and when decided to limit signal level
of said transmission signal, outputs a coefficient which limits signal
level of said transmission signal to no more than the first limit value;a
signal level limitation unit that limits signal level of said
transmission signal by multiplying said coefficient to said transmission
signal;a distortion compensation coefficient calculation unit that
obtains a difference between a signal before compensation which is an
output signal of said signal level limitation unit and a feedback signal
which fed back part of output signals of an amplifier which amplified a
signal after distortion compensation which is a signal such that
distortion compensation processing is performed to a signal before
compensation, and calculates an amplitude compensation coefficient and a
phase compensation coefficient;a storage unit that stores said amplitude
compensation coefficient and said phase compensation coefficient by
correlating them to electric power or amplitude of said signal before
compensation;a distortion compensation operation unit that performs said
distortion compensation processing to said signal before compensation
based on said amplitude compensation coefficient and a phase compensation
coefficient, and outputs said signal after distortion compensation, anda
limit value calculation unit that calculates signal level of said signal
before distortion compensation as said first limit value providing the
signal level after said distortion compensation processing based on said
amplitude compensation coefficient and said phase compensation
coefficient stored in said storage unit is no more than a second limit
value which is a limit value of signal level of an input signal to an
amplifier.

20. A distortion compensation method comprising:limiting signal level of
an input signal from outside to no more than a first limit value, and
outputting a signal before distortion compensation;based on a distortion
compensation coefficient, performing distortion compensation processing
to said signal before distortion compensation, and outputting a signal
after distortion compensation;based on a difference between part of
output signals of an amplifier which amplified said signal after
distortion compensation and said signal before distortion compensation,
calculating a coefficient for compensating distortion of an output signal
of said amplifier as said distortion compensation coefficient;storing
said distortion compensation coefficient by correlating it to signal
level of said signal before distortion compensation, andcalculating
signal level of said signal before distortion compensation as said first
limit value providing the signal level after said distortion compensation
processing based on said distortion compensation coefficient stored in
said storage unit is no more than a second limit value which is a limit
value of signal level of an input signal to said amplifier.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a distortion compensation circuit
and a distortion compensation method and, in particular, relates to a
distortion compensation circuit and a distortion compensation method for
outputting a signal with predetermined compensation applied to an
amplifier so that distortion does not occur to the output signal of the
amplifier.

BACKGROUND ART

[0002]In a digital wireless communication system of recent years,
multiplexing by a CDMA (Code Division Multiple Access) method or
modulation by an OFDM (Orthogonal Frequency Division Multiplexing) method
is used. A multiple modulated wave generated in some sort of
communication systems such as a communication system using a CDMA method
or an OFDM method has a characteristic to include a wave with
instantaneous electric power much larger than average electric power.
Therefore, a power amplifier for transmission of the above-mentioned
communication systems needs to maintain linearity up to very high output
level. This is because, suppressing the spread of a transmission spectrum
caused by nonlinear distortion reduces adjacent channel leakage power.
Further, instantaneous electric power is expressed using a peak factor or
a crest factor and so on.

[0003]However, there is a problem that a power amplifier having good
linearity up to very high output level, that is, up to a large amplitude
component, has a large circuit scale, is expensive, and also has large
power consumption. Therefore, a power amplifier having good linearity in
a small amplitude component, but having nonlinearity in a large amplitude
component is generally well used. For a power amplifier having such
nonlinearity, by making an output back-off as small as possible, further
efficiency improvement can be implemented. The back-off is to lower
average electric power to prevent saturation and so on. However, when the
back-off is decreased, because an input signal tends to enter a
nonlinearity region of an amplifier, adjacent channel leakage power which
is a distortion component of a wireless transmission signal is apt to
become large.

[0004]In order to suppress adjacent channel leakage power caused by
nonlinear distortion of a power amplifier, various distortion
compensation technologies are proposed. Among them, as a distortion
compensation technology adopted most often in recent years, there is a
digital pre-distortion method.

[0006]The nonlinear distortion compensation operation unit 4 performs
distortion compensation operation for digital quadrature baseband signals
I and Q from transmission data generation unit 21 by complex
multiplication based on distortion compensation coefficients K and
θ calculated in advance. Quadrature baseband signals I'' and Q''
after distortion compensation operation is performed, are converted by
the DACs 22A and 22B into analog signals and become analog quadrature
baseband signals. And the quadrature modulator 23 converts analog
quadrature baseband signals into quadrature modulation signals using a
signal from the reference signal generator unit 24. Quadrature modulation
signals are power amplified by the power amplifier 25, and outputted as
an RF (Radio Frequency) signal.

[0007]Part of the RF signals are fed back to the quadrature demodulator 27
via the directional coupler 26, and are demodulated to analog quadrature
baseband signals using a signal from the reference signal generator unit
24. Analog quadrature baseband signals are converted by the ADCs 28A and
28B into digital signals and become feedback quadrature baseband signals
Ib and Qb.

[0008]The distortion compensation coefficient update unit 5A compares
feedback quadrature baseband signals Ib and Qb and quadrature baseband
input signals I and Q from the transmission data generation unit 21 and
calculates a distortion compensation coefficient. The distortion
compensation coefficient update unit 5A updates the distortion
compensation coefficient to the latest value and stores it. At this time,
the distortion compensation coefficient update unit 5A stores the
distortion compensation coefficient by correlating it to electric power
of quadrature baseband signals I and Q. Electric power of quadrature
baseband signals I and Q is calculated by the electric power computation
unit 8.

[0010]The circuit configuration of a distortion compensation circuit of a
digital pre-distortion method described above is one example. A circuit
configuration which adopts digital method for quadrature modulation and
quadrature demodulation or a circuit configuration using a frequency
converter instead of direct modulation is also proposed. Also, a circuit
configuration which performs distortion compensation operation using a
distortion compensation coefficient correlated to amplitude instead of a
distortion compensation coefficient correlated to electric power is also
proposed. Further, amplitude is proportional to a square root of electric
power. Accordingly, by setting up amplitude computation unit which
computes amplitude, amplitude can be obtained based on electric power.

[0011]An example of an input/output characteristic of a wireless
transmitter including a distortion compensation circuit of the digital
pre-distortion method mentioned above is shown in FIG. 6. FIG. 6
indicates input/output characteristics of the nonlinear distortion
compensation operation unit 4, the power amplifier 25 and an entire
wireless transmitter respectively per input level normalized by setting
saturated input level of the power amplifier 25 as 1. It can be found
that, by giving an inverse characteristic of the input/output
characteristic of the power amplifier 25 having nonlinearity in the
nonlinear distortion compensation operation unit 4, a linear input/output
characteristic is obtained for the entire wireless transmitter.

[0012]However, as the input/output characteristic of the nonlinear
distortion compensation operation unit 4 indicates, when distortion
compensation is made, at an input level of about 0.5-0.6, which is about
4 dB-6 dB lower than 1 which is saturated input level, signal level after
distortion compensation operation reaches a saturated point of the power
amplifier 25. In case of no compensation, a phenomenon such as a range of
input level which can be inputted without making the power amplifier 25
being saturated is limited to a range with a minimum below the saturated
input level does not occur normally.

[0013]And an amount for which input level is limited against saturated
input level is equal to a decreased amount of an actual gain against a
linear gain of a saturated output point. In other words, a limited amount
of input level and a decreased amount of linear gain are equal. This
means that, for a distortion compensation circuit of a digital
pre-distortion method of FIG. 5, a relation of output back-off=input
back-off holds. As mentioned above, for the distortion compensation
circuit of a digital pre-distortion method of FIG. 5, in order to avoid
the generation of clipping distortion in a saturated region of an
amplifier, input level needs to be limited. Further, although occurrence
of clipping distortion can be prevented by limiting input level, a
distortion compensation circuit of a pre-distortion method cannot
compensate clipping distortion which occurred once.

[0014]The limitations mentioned above relating to a distortion
compensation circuit of a pre-distortion method also apply similarly to
an instantaneous input. That is, by performing distortion compensation,
an output of a power amplifier reaches a saturated region for
instantaneous input level also, which is several dB lower than saturated
input level of in case of no compensation. Therefore, there is a problem
that clipping distortion increases and distortion compensation effect
declines.

[0015]Further, "clipping distortion" is a ratio of leakage power at
frequency deviated for certain frequency from center frequency, which is
generated when a signal is inputted to an amplifier with an ideal
amplifier characteristic having an ideal limiter characteristic, against
electric power at center frequency. Further, "ideal limiter
characteristic" is a characteristic by which an amplitude-amplitude
characteristic (hereinafter called as "AM (Amplitude Modification)/AM
characteristic") is linear up to a saturated point. "Ideal amplifier
characteristic" is a characteristic which has "ideal limiter
characteristic" and whose amplitude-phase characteristic (hereinafter
called as "AM/PM (Phase Modification) characteristic" is flat.

[0016]Thus, distortion compensation by a digital pre-distortion method
compensates only nonlinear distortion of AM/AM characteristics and AM/PM
characteristics generated by a power amplifier. Distortion compensation
by a digital pre-distortion method cannot compensate clipping distortion
in a saturated region. As mentioned above, a multiplex modulated wave
which is generated in a communication system using a CDMA method or in a
communication system using OFDM modulation and so on includes a wave with
instantaneous power whose amplitude is very large against average
electric power. Accordingly, as clipping distortion is apt to occur, it
is important to limit instantaneous maximum electric power of an input
signal not to exceed the saturated electric power of a power amplifier
for transmission.

[0017]Further, distortion compensation equipment by a pre-distortion
method is disclosed, for example, in Japanese Patent Application
Laid-Open No. 2002-223171. A method to suppress amplitude adaptively is
disclosed, for example, in Japanese Patent Application Laid-Open No.
2004-064711.

[0018]A distortion compensation circuit for suppressing very large
instantaneous amplitude which occurs on an envelope, and which becomes a
factor of the above-mentioned problem, is disclosed in Japanese Patent
Application Laid-Open No. 2001-251148 (hereinafter referred to as "patent
document 1") and Japanese Patent Application Laid-Open No. 2003-168931
(hereinafter referred to as "patent document 2").

[0021]The distortion compensation coefficient calculation unit 6 compares
quadrature baseband signals I and Q with feedback quadrature baseband
signal Ib and Qb on the polar coordinates, calculates an amplitude
difference and a phase difference, and calculates distortion compensation
coefficient h based on them. The coefficient limitation determination
unit 13 computes, using distortion compensation coefficient h calculated
in the distortion compensation coefficient calculation unit 6, electric
power x of a signal after distortion compensation operation to inputted
quadrature baseband signals I and Q is performed. By comparing electric
power x and upper limit electric power Pmax set to the limit value
setting unit 16 in advance, it is judged whether limitation is given to a
distortion compensation coefficient. And determination result and
calculated electric power x after distortion compensation operation are
outputted to the distortion compensation coefficient correction unit 12.

[0022]In case electric power x after distortion compensation operation is
no more than upper limit electric power Pmax, limitation is not given to
distortion compensation coefficient h. In this case, the distortion
compensation coefficient correction unit 12 outputs distortion
compensation coefficient h calculated in the distortion compensation
coefficient calculation unit 6 to the distortion compensation coefficient
data memory 7 just as it is.

[0023]On the other hand, when electric power x after distortion
compensation operation exceeds upper limit electric power Pmax,
limitation is given to distortion compensation coefficient h. In this
case, the distortion compensation coefficient correction unit 12, after
making a correction that distortion compensation coefficient h calculated
in the distortion compensation coefficient calculation unit 6 becomes 1/m
times as much, outputs it to the distortion compensation coefficient data
memory 7. Here, m is an amplitude limitation coefficient, and is
calculated using the following formula:

m=(x/Pmax)1/2.

[0024]The distortion compensation coefficient data memory 7 stores the
distortion compensation coefficient h by correlating it to electric power
of quadrature baseband signals I and Q from the electric power
computation unit 8.

[0025]Here, by using a distortion compensation circuit including the
distortion compensation coefficient update unit 5B as shown in FIG. 7, it
is possible to correct the size of an amplitude compensation coefficient
while to maintaining the phase to an input signal, and to limit signal
amplitude after distortion compensation operation.

[0026]In the technology described in patent document 1, because limitation
is being made to the amplitude compensation coefficient itself, amplitude
of feedback quadrature baseband signals Ib and Qb from the power
amplifier 25 (refer to FIG. 5) is limited. However, amplitude of
quadrature baseband signals I and Q is not limited. That is, in the
distortion compensation coefficient calculation unit 6, a distortion
compensation coefficient is calculated by comparison operation between
quadrature baseband input signals I and Q for which amplitude is not
limited and feedback quadrature baseband signals Ib and Qb for which
amplitude is limited. Therefore, an amplitude difference and a phase
difference between feedback quadrature baseband signals Ib and Qb, and I
and Q cannot be obtained correctly. Accordingly, there is a problem that
highly precise distortion compensation cannot be implemented.

[0027]As a method of resolution for this problem, there is a method
disclosed in patent document 2. The method of patent document 2 utilizes
the fact that, when input amplitude does not exceed the distortion
compensable upper limit input amplitude (in an example of FIG. 6, about
0.5-0.6) of the nonlinear distortion compensation operation unit 4,
amplitude after distortion compensation operation does not exceed
saturated input amplitude of a power amplifier. In other words, the
method disclosed in patent document 2 is a method to limit a maximum
value of amplitude based on the relation of output back-off=input
back-off. That is, in a preceding step of a circuit in which distortion
compensation operation is made, a maximum value of amplitude is limited
to the level in which input amplitude will be the amplitude where output
back-off is added to an effective value.

[0028]FIG. 8 is a block diagram showing one exemplary structure of a
wireless transmitter described in patent document 2. Further, in FIG. 8,
an identical code is attached to a component identical to the component
shown in FIG. 5. Distortion compensation circuit 30 of FIG. 8 includes
electric power computation unit 1, coefficient calculation unit 14, limit
value setting unit 15 and amplitude limitation unit 3 in the receding
step of the nonlinear distortion compensation operation unit 4, and other
components are same as FIG. 5. Further, in FIG. 8, part of the names of
components in the drawings disclosed in patent document 2 are simplified
or modified.

[0029]The electric power computation unit 1 computes electric power P of
quadrature baseband signals I and Q and outputs electric power P to the
coefficient calculation unit 14. The coefficient calculation unit 14
compares electric power P and electric power limit value Pth set to the
limit value setting unit 15 in advance. The coefficient calculation unit
14 decides not to perform amplitude limitation in case the electric power
P is no more than electric power limit value Pth, and outputs 1 as a
multiplier coefficient. On the other hand, in case the electric power P
is larger than the electric power limit value Pth, the coefficient
calculation unit 14 decides to perform amplitude limitation, and output
(Pth/P)1/2 as a multiplier coefficient. Further, a multiplier
coefficient when amplitude limitation is performed is not limited to this
value, and any value such that electric power value P of quadrature
baseband signals I and Q become not more than limit value Pth will do.

[0030]The amplitude limitation unit 3 performs amplitude limitation by
"circular clipping" by multiplying a multiplier coefficient from the
coefficient calculation unit 14 to each of an I component and a Q
component of a quadrature baseband signal from transmission data
generation unit 2. The "circular clipping" is a clipping where the
amplitude which is a result of an I component and a Q component being
synthesized is limited to no more than a fixed size for all phases. The
nonlinear distortion compensation operation unit 4 performs distortion
compensation by complex multiplication based on a distortion compensation
coefficient, to quadrature baseband signals I' and Q' after amplitude
limitation for the signals is performed. An updating method of a
distortion compensation coefficient and a referring method of a
distortion compensation coefficient are similar to an example mentioned
above.

[0031]Thus, in the distortion compensation circuit described in patent
document 2, nonlinear distortion compensation is performed, by a circular
clipping on the orthogonal coordinates, after performing amplitude
limitation of a quadrature baseband transmission signal. Accordingly, the
distortion compensation circuit described in patent document 2 will not
err in operations to obtain an amplitude difference and a phase
difference, suppress clipping distortion component and can improve the
later nonlinear distortion compensation effect substantially.

DISCLOSURE OF THE INVENTION

Technical Problem

[0032]However, the above-mentioned technologies include the following
problems respectively. In the technology described in patent document 2,
when a characteristic of the power amplifier 25 is always constant, a
problem about distortion compensation will not occur. However, perfect
distortion compensation operation is not guaranteed case saturated output
level of the power amplifier 25 declines by aged deterioration or
temperature change (output back-off declines) or in case a breakdown of
the power amplifier 25 occurs.

[0033]For example, when saturated output level of the power amplifier 25
declines, the distortion compensable upper limit input level of the
nonlinear distortion compensation operation unit 4 shown in FIG. 6 also
declines. At this time, as electric power limit value Pth of the limit
value setting unit 15 is not changed, signal level after amplitude
limitation exceeds the distortion compensable upper limit input level of
the nonlinear distortion compensation operation unit 4. And because no
limitation is made to amplitude for a signal after distortion
compensation operation, clipping distortion is generated. Also, when
saturated output level of the power amplifier 25 further declines, and a
distortion compensation coefficient has diverged, there is a risk that
very large signal amplitude is inputted to a power amplifier, and the
power amplifier may break down.

[0034]As stated above, in order to perform perfect distortion
compensation, amplitude limitation to a quadrature baseband signal by the
technology described in patent document 2 only is not sufficient.
Accordingly, because parallel use with a technology which applies
limitation to signal level after distortion compensation or to a
distortion compensation coefficient itself, for example, the technology
described in patent document 1, is needed, after all, a problem which
cannot calculate a highly precise distortion compensation coefficient is
not resolved.

[0035]The present invention is made in view of a problem of the
above-mentioned related technologies, and the object is to provide a
distortion compensation circuit and a distortion compensation method
which limit signal level of a signal after distortion compensation, and
improve distortion compensation effect.

Technical Solution

[0036]A distortion compensation circuit of the present invention includes:
an input level limitation means which limits signal level of an input
signal from outside to no more than a first limit value, and outputs a
signal before distortion compensation; a distortion compensation means
which, based on a distortion compensation coefficient, performs
distortion compensation processing to a signal before distortion
compensation and outputs a signal after distortion compensation; a
distortion compensation coefficient calculation means which, based on a
difference between part of output signals of an amplifier which amplified
signals after distortion compensation and signals before distortion
compensation, calculates a coefficient for compensating distortion of the
output signal of the amplifier as a distortion compensation coefficient;
a storage means which, by correlating it to the signal level of the
signal before distortion compensation, stores the distortion compensation
coefficient, and a limit value calculation means which, based on the
distortion compensation coefficient stored in the storage means,
calculates signal level of a signal before distortion compensation when
signal level while distortion compensation processing is performed is no
more than a second limit value which is a limit value of signal level of
an input signal to an amplifier as the first limit value.

[0037]A distortion compensation method of the present invention includes:
a step which limits signal level of an input signal from outside to no
more than a first limit value, and outputs a signal before distortion
compensation; a step which, based on a distortion compensation
coefficient, performs distortion compensation processing to a signal
before distortion compensation and outputs a signal after distortion
compensation; a step which, based on a difference between part of output
signals of an amplifier which amplified signals after distortion
compensation and signals before distortion compensation, calculates a
coefficient for compensating distortion of the output signal of the
amplifier as a distortion compensation coefficient, a step which, by
correlating it to the signal level of the signal before distortion
compensation, stores the distortion compensation coefficient, and a step
which, based on the distortion compensation coefficient stored in the
storage means, calculates signal level of a signal before distortion
compensation when signal level while distortion compensation processing
is performed is no more than a second limit value which is a limit value
of signal level of an input signal to an amplifier as the first limit
value.

ADVANTAGEOUS EFFECT OF THE INVENTION

[0038]A distortion compensation circuit and a distortion compensation
method of the present invention have the effect to improve the distortion
compensation effect.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039]FIG. 1 is a schematic block diagram of a distortion compensation
circuit of the first embodiment of the present invention.

[0040]FIG. 2 is a block diagram showing a structure of a distortion
compensation circuit of the second exemplary embodiment of the present
invention.

[0041]FIG. 3 is a block diagram showing a structure of a distortion
compensation circuit of the third exemplary embodiment of the present
invention.

[0042]FIG. 4 is a block diagram showing a structure of a distortion
compensation circuit of the fourth exemplary embodiment of the present
invention.

[0043]FIG. 5 is a block diagram showing an exemplary structure of a
wireless transmitter including a distortion compensation circuit of a
digital pre-distortion method.

[0044]FIG. 6 is an explanatory drawing showing operation of a distortion
compensation circuit of a digital pre-distortion method.

[0045]FIG. 7 is a block diagram showing an exemplary structure of the
distortion compensation coefficient update unit described in patent
document 1.

[0046]FIG. 8 is a block diagram showing an exemplary structure of the
wireless transmitter described in patent document 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The First Exemplary Embodiment

[0047]The first exemplary embodiment of the present invention will be
described with reference to drawings. FIG. 1 is a schematic block diagram
of a distortion compensation circuit of the first exemplary embodiment of
the present invention. This exemplary embodiment indicates a principle of
the present invention in order to help understanding of the present
invention.

[0049]The coefficient calculation means 201 judges whether to perform
amplitude limitation of quadrature baseband transmission signals I and Q
by comparing electric power value of quadrature baseband transmission
signals I and Q and electric power limit value before compensation which
will be described later. And the coefficient calculation means 201, when
decided to perform amplitude limitation, outputs a coefficient for making
electric power of quadrature baseband transmission signals I and Q not
being more than an electric power limit value before compensation as a
multiplier coefficient.

[0052]A distortion compensation coefficient can be obtained, for example,
by adaptive signal processing using LMS (Least Mean Square) algorithm
disclosed in patent document 1. That is, by comparing a signal before
distortion compensation and a signal after distortion compensation
amplified by an amplifier, amplitude compensation coefficient K and phase
compensation coefficient θ are calculated so that the difference
between the compared signals will be zero. Signals before distortion
compensation correspond to output signals I' and Q' of the amplitude
limitation means 202, and signals after distortion compensation amplified
by an amplifier correspond to feedback quadrature baseband signals Ib and
Qb. Further, a calculation method of a distortion compensation
coefficient is not limited to a method using LMS algorithm. In short, a
calculation method of a distortion compensation coefficient can be a
method that calculates a distortion compensation coefficient to minimize
the difference between a signal before distortion compensation and a
signal after distortion compensation amplified by an amplifier.

[0053]The storage means 205, by correlating it with electric power or
amplitude of output signals I' and Q' of the amplitude limitation means
202, stores amplitude compensation coefficient K and phase compensation
coefficient θ. For example, a value of electric power or amplitude
of output signals I' and Q' are made an address of a storage area set up
in storage means 205. And as data, amplitude compensation coefficient K
and phase compensation coefficient θ corresponding to output
signals I' and Q' are stored at the address of the storage area.

[0055]The limit value calculation means 206 refers to stored contents of
the storage means 205, and obtains electric power in case distortion
compensation operation to output signals I' and Q' is performed by the
distortion compensation operation means 203. And electric power of output
signals I' and Q' when the electric power becomes nearly equal to an
electric power limit value after compensation, is calculated as the
electric power limit value before compensation mentioned above.

[0056]In other words, electric power of a signal after distortion
compensation operation when distortion compensation operation is
performed to output signals I' and Q' which are signals before distortion
compensation operation, is obtained. At this time, output signals I'' and
Q'' are not outputted from the distortion compensation operation means
203 actually, but in every respect, electric power of a signal after
distortion compensation operation when it is assumed that distortion
compensation operation by the distortion compensation operation means 203
is performed, is obtained. And electric power of output signals I' and Q'
for which electric power of a signal after distortion compensation
operation becomes nearly equal to an electric power limit value after
compensation, is set as an electric power limit value before
compensation. Accordingly, an electric power limit value before
compensation is the limit value for limiting electric power of output
signals P and Q' so that electric power of a signal after distortion
compensation operation will be no more than the electric power limit
value after compensation when distortion compensation operation is
performed.

[0057]Thus, the limit value calculation means 206 calculates the first
limit value for performing amplitude limitation in the coefficient
calculation means 201 based on stored contents of the storage means 205.
Accordingly, amplitude limitation to quadrature baseband transmission
signals I and Q is performed adaptively, and amplitude of a signal after
distortion compensation operation is limited to no more than the second
limit value.

[0058]Further, the coefficient calculation means 201 may judge whether to
perform amplitude limitation by comparing amplitude of quadrature
baseband transmission signals I and Q with an amplitude limit value which
is a limit value to amplitude instead of electric power of quadrature
baseband IS transmission signals I and Q. When decided to limit
amplitude, the coefficient calculation means 201 outputs a coefficient
for making amplitude of quadrature baseband transmission signals I and Q
not more than the amplitude limit value as a multiplier coefficient.

[0059]The limit value calculation means 206 refers to a distortion
compensation coefficient stored in the storage means 205, and calculates
amplitude of output signals I' and Q' such that amplitude when distortion
compensation operation to output signals I' and Q' of the amplitude
limitation means 202 is performed becomes nearly equal to the amplitude
limit value after compensation as an amplitude limit value.

[0060]As stated above, in a distortion compensation circuit of the first
exemplary embodiment, determination is made whether to limit amplitude of
an input signal based on the electric power limit value or the amplitude
limit value. The electric power limit value or the amplitude limit value
is decided according to the signal level limit value after compensation.
Therefore, electric power or amplitude of a signal after distortion
compensation operation is limited to no more than the signal level limit
value after compensation. Accordingly, because electric power or
amplitude of a signal inputted to an amplifier is limited to no more than
the predetermined limit value, there is an effect that can improve the
effect of the distortion compensation.

The Second Exemplary Embodiment

[0061]Next, the second exemplary embodiment having an indispensable
component only to put the present invention into effect will be
described. FIG. 2 is a block diagram showing a structure of a distortion
compensation circuit of the second exemplary embodiment of the present
invention. As shown, a distortion compensation circuit of this exemplary
embodiment is a distortion compensation circuit of the first exemplary
embodiment with the coefficient calculation means 201 removed. Also,
quadrature baseband transmission signals I and Q in the first exemplary
embodiment are generalized to a normal signal of a single system.

[0062]A distortion compensation circuit of this exemplary embodiment
performs predetermined distortion compensation processing to an output
signal, that is, an input signal to an amplifier in advance, in order to
suppress distortion which occurs to an output signal of an amplifier
which amplifies a certain input signal. This distortion compensation
method is the above-mentioned pre-distortion method.

[0064]The input level limitation means 301 is a generalized amplitude
limitation means 202. The input level limitation means 301 limits input
level of input signal 306, by a predetermined method, to no more than the
input level limit value described later. For example, input level is
limited by multiplying a predetermined coefficient to the input signal
306. Further, similar to the first exemplary embodiment, only when input
level of the input signal 306 is no smaller than the input level limit
value, input level may be limited.

[0065]Further, limitation to an input signal may not only be limitation to
amplitude like the first exemplary embodiment but also be limitation to
"signal level" in general. The "signal level" is an amount representing
the size of a signal which is expressed by amplitude measured by volt or
electric power measured by watt or dBm and so on.

[0066]The input level limitation means 301 limits signal level of the
input signal 306 and inputs signal before distortion compensation 307 to
the distortion compensation means 302. Conditions to limit signal level
and a degree of limitation of the signal level will be described later.

[0067]The distortion compensation means 302 is a generalized distortion
compensation operation means 203. The distortion compensation means 302
performs distortion compensation to the signal before distortion
compensation 307 using a distortion compensation coefficient described
later. An actual signal processing method for distortion compensation to
be performed to the signal before distortion compensation 307 is not
limited to multiplication of a distortion compensation coefficient to the
signal before distortion compensation 307. For example, a table lookup
method using a reference table which relates an input signal to a signal
after distortion compensation and so on may be used. The distortion
compensation means 302 outputs signal after distortion compensation 308
to an external amplifier (not illustrated). The external amplifier
amplifies the signal after distortion compensation 308 and outputs an
amplifier output signal.

[0068]The distortion compensation coefficient calculation means 303
performs similar operation to the distortion compensation coefficient
calculation means 204. That is, the distortion compensation coefficient
calculation means 303 obtains a difference between the signal before
distortion compensation 307 and feedback signal 309 which is part of
amplifier output signals fed back, and calculates a distortion
compensation coefficient based on the difference. For example,
differences are an amplitude difference or a phase difference.

[0069]The storage means 304 also performs similar operation to the storage
means 205. That is, the storage means 304 stores a distortion
compensation coefficient by correlating it to signal level of the signal
before distortion compensation 307.

[0070]The input level limit value calculation means 305 is a generalized
limit value calculation means 206. That is, the input level limit value
calculation means 305 changes the input level limit value for limiting
input level in the input level limitation means 301 based on a distortion
compensation coefficient stored in storage means 304. The input level
limit value calculation means 305 refers to a distortion compensation
coefficient stored in the storage means 304 and obtains the signal level
when the signal before distortion compensation 307 is assumed to be
distortion compensated by the distortion compensation means 302. And in
order that the signal level will be no more than the signal level limit
value after compensation set to the signal after distortion compensation
308, a limit value of input level which should be applied to the input
signal 306, that is, the input level limit value mentioned above will be
obtained. The signal level limit value after compensation is, for
example, signal level of the signal after distortion compensation 308
which will not make an output signal of an amplifier saturated.

[0071]Further, when the input level limit value calculation means 305
calculates an input level limit value, the signal after distortion
compensation 308 is not outputted. In every respect, signal level of a
signal after distortion compensation when signal before distortion
compensation 307 is assumed to be distortion compensated, is obtained,
and based on the signal level, a limit value of input level is obtained.

[0072]As stated above, in a distortion compensation circuit of the second
exemplary embodiment determines whether to limit signal level of an input
signal based on the input level limit value. The input level limit value
is decided according to the signal level limit value after compensation.
Therefore, signal level of a signal after distortion compensation is
limited to no more than the signal level limit value after compensation.
Accordingly, because signal level of a signal inputted to an amplifier is
limited to no more than the signal level limit value after compensation,
there are, for example, effects that no clipping distortion is generated
in an output signal and distortion compensation effects can be improved.

The Third Exemplary Embodiment

[0073]Hereinafter, the third exemplary embodiment of the present invention
will be described with reference to drawings. FIG. 3 is a block diagram
showing a structure of a distortion compensation circuit of the third
exemplary embodiment of the present invention. For those components in
FIG. 3 equal to those in FIG. 5 and FIG. 8, an identical code is
attached. A distortion compensation circuit of this exemplary embodiment
is used in a wireless transmitter such as a base station in a wireless
communication system and is a distortion compensation circuit for
compensating distortion that occurs in an amplifying means such as a
power amplifier. This distortion compensation circuit includes electric
power computation unit 1, coefficient calculation unit 2, amplitude
limitation unit 3, nonlinear distortion compensation operation unit 4,
distortion compensation coefficient update unit 5A, electric power
computation unit 8, input electric power limit value decision unit 9,
limit value setting unit 10 and compensation upper limit input electric
power calculation unit 11. The distortion compensation coefficient update
unit 5A includes distortion compensation coefficient calculation unit 6
and distortion compensation coefficient data memory 7.

[0074]The distortion compensation circuit of this exemplary embodiment
limits amplitude based on the limit value decided adaptively to digital
quadrature baseband signals I and Q. As an amplitude limitation method,
for example, an amplitude limitation method by a circular clipping or an
amplitude limitation method using a window function can be used.

[0075]In an amplitude limitation method by a circular clipping, electric
power of quadrature baseband signals I and Q is calculated, and for a
signal exceeding an electric power limit value, amplitude is limited
while maintaining the phase of the signal. Although an amplitude
limitation method by a circular clipping is a simple method, limitation
processing is performed only for a part exceeding the limit value.
Therefore, discontinuity occurs in a processing result before and after
the limitation point.

[0076]On the other hand, in an amplitude limitation method using a window
function, when electric power x of quadrature baseband signals I and Q is
larger than electric power limit value y, a window function for limiting
electric power x to no more than electric power limit value y is
generated. Window function is a function whose function value is 0
outside of a certain limited interval. And a window function having a
fixed time interval is multiplied to a signal with electric power
exceeding electric power limit value y and to a signal with electric
power in the neighborhood of electric power limit value y. As a window
function, well-known Hanning window or Kaisar window can be used.
Although this processing is complicated, discontinuity will not occur.
Either of the amplitude limitation methods mentioned above may be used.
Below, a case will be described when an amplitude limitation method by a
circular clipping is used.

[0077]The electric power computation unit 1 computes electric power P
(=12+Q2) of quadrature baseband signals I and Q and outputs it
to the coefficient calculation unit 2. The coefficient calculation unit 2
compares electric power P from the electric power computation unit 1 and
electric power limit value Pth given from the input electric power limit
value decision unit 9. In case electric power P is no more than electric
power limit value Pth, the coefficient calculation unit 2 decides not to
perform amplitude limitation, and outputs I as a multiplier coefficient
in order not to limit quadrature baseband signals I and Q. On the other
hand, in case electric power P is larger than electric power limit value
Pth, the coefficient calculation unit 2 decides to perform amplitude
limitation and outputs (Pth/P)1/2 as a multiplier coefficient.
Further, a multiplier coefficient when amplitude limitation is performed
is not limited to this value, and any value such that electric power P of
quadrature baseband signals I and Q becomes not more than electric power
limit value Pth will do.

[0078]The amplitude limitation unit 3 performs amplitude limitation by a
circular clipping by multiplying a multiplier coefficient from the
coefficient calculation unit 2 to each of an I component and a Q
component of a quadrature baseband signal. The nonlinear distortion
compensation operation unit 4 performs distortion compensation by complex
multiplication based on distortion compensation coefficients K and
θ from the distortion compensation coefficient update unit 5A to
quadrature baseband signals I' and Q' after amplitude limitation is
performed. And the nonlinear distortion compensation operation unit 4
outputs quadrature baseband signals I'' and Q'' after distortion
compensation is performed, to an analog signal processing unit. An analog
signal processing unit is, for example, a circuit including the DACs 22A
and 22B, quadrature modulator 23 and power amplifier 25 in FIG. 5.

[0080]Further, feedback quadrature baseband signals Ib and Qb are
generated based on a signal which is part of output signals of a power
amplifier which is at the following step of a distortion compensation
circuit fed back to the quadrature demodulator 27 via the directional
coupler 26. A signal which is fed back to the quadrature demodulator 27
will be demodulated to an analog quadrature baseband signal by a signal
from the reference signal generator unit 24, be converted into a digital
signal afterward and become quadrature baseband signals Ib and Qb (refer
to FIG. 5 and FIG. 8).

[0081]The electric power computation unit 8 computes electric power P'
(=I'2+Q'2) of quadrature baseband signals I' and Q' for which
amplitude limitation was performed and outputs it to the distortion
compensation coefficient data memory 7. Further, instead of electric
power P' of quadrature baseband signals I' and Q', amplitude proportional
to square root of electric power P' may be used. In this case, the
distortion compensation coefficient data memory 7 stores the distortion
compensation coefficients K and θ by correlating them to amplitude.

[0086]The input electric power limit value decision unit 9 outputs decided
electric power limit value Pth to the coefficient calculation unit 2.
Also, by obtaining several electric power limit value Pth, their average
may be taken and outputted to the coefficient calculation unit 2. That
is, the input electric power limit value decision unit 9 may calculate
reasonable number of electric power limit value Pth chronologically,
store them, take average Pth (ave) of multiple calculated Pth, and output
it to the coefficient calculation unit 2.

[0087]Upper limit electric power Pmax fixed in advance is set in the limit
value setting unit 10. It is desirable that a value of upper limit
electric power Pmax is set to input level which makes output of a power
amplifier saturated, that is, a value equal to saturated input level.

[0088]Further, the upper limit electric power Pmax is not necessarily set
to a value equal to saturated input level. For example, considering
reduction in power consumption or elongation of a life cycle of a device
and so on, the level inputted to an amplifier may be set lower than
saturated input level. In such a case, a value of the upper limit
electric power Pmax may be set to a value smaller than saturated input
level.

[0089]The compensation upper limit input electric power calculation unit
11 refers to the distortion compensation coefficient data memory 7, and
calculates input electric power Plim such that electric power of a
quadrature baseband signal after distortion compensation operation
becomes upper limit electric power Pmax (saturated input level) set by
the limit value setting unit 10. In other words, if amplitude
compensation coefficient K to input electric power Plim is α, then,
calculate input electric power Plim for which the formula

Pmax=α×Plim

[0090]holds. Input electric power Plim calculated in this way is
distortion compensable upper limit input electric power. Because the
distortion compensation coefficient data memory 7 stores a distortion
compensation coefficient by correlating it to input electric power,
α×Plim can be obtained easily. Accordingly, input electric
power Plim such that α×Plim becomes equal to Pmax can also be
obtained easily.

[0091]Next, operation of a distortion compensation circuit by the third
embodiment of the present invention will be described with reference to
FIG. 3. The coefficient calculation unit 2 judges whether to perform
amplitude limitation by comparing electric power P of digital quadrature
baseband signals I and Q calculated in the electric power computation
unit 1 with electric power limit value Pth given from input electric
power limit value decision unit 9. In case electric power P is no more
than electric power limit value Pth, the coefficient calculation unit 2
decides not to perform amplitude limitation, and outputs 1 to the
amplitude limitation unit 3 as a multiplier coefficient. On the other
hand, in case electric power P is larger than electric power limit value
Pth, the coefficient calculation unit 2 decides to perform amplitude
limitation and outputs (Pth/P)1/2 to the amplitude limitation unit 3
as a multiplier coefficient.

[0092]The amplitude limitation unit 3 performs amplitude limitation by a
circular clipping by multiplying a multiplier coefficient from the
coefficient calculation unit 2 to each of an I component and a Q
component of a quadrature baseband signal. Here, instead of a circular
clipping, an amplitude limitation method using a window function may be
applied. In this case, when electric power P is larger than electric
power limit value Pth, the coefficient calculation unit 2 generates a
window function convex cup with (Pth/P)1/2 as an apex. The amplitude
limitation unit 3 multiplies the window function mentioned above having a
fixed time interval to an electric power point which exceeds the limit
value of quadrature baseband signals I and Q and an electric power point
in the neighborhood.

[0093]Quadrature baseband signals I' and Q' for which amplitude limitation
was performed are inputted to the nonlinear distortion compensation
operation unit 4. The nonlinear distortion compensation operation unit 4
refers to distortion compensation coefficients K and θ
corresponding to electric power P' of quadrature baseband signals I' and
Q'. For example, taking values of I' and Q' as an address, reads
distortion compensation coefficients K and θ from the distortion
compensation coefficient data memory 7. And the nonlinear distortion
compensation operation unit 4 performs distortion compensation operation
by complex multiplication based on distortion compensation coefficients K
and θ read out to baseband signals I' and Q', and outputs them to
an analog signal processing unit. An analog signal processing unit is,
for example, a circuit including the DACs 22A and 228, the quadrature
modulator 23 and the power amplifier 25 in FIG. 5.

[0094]Part of the output of a power amplifier included in an analog signal
processing unit is converted into a digital signal, and fed back to the
distortion compensation coefficient calculation unit 6 as feedback
quadrature baseband signals Ib and Qb. The distortion compensation
coefficient calculation unit 6 compares feedback quadrature baseband
signals Ib and Qb and quadrature baseband input signals I' and Q' on the
polar coordinates and calculates an amplitude difference and a phase
difference. Then the distortion compensation coefficient calculation unit
6 calculates distortion compensation coefficients K and θ based on
the amplitude difference and the phase difference, and updates a value of
the distortion compensation coefficient data memory 7 by correlating it
to electric power P' computed in electric power computation unit 8. The
nonlinear distortion compensation operation unit 4 performs adaptive
distortion compensation by performing distortion compensation based on
the updated distortion compensation coefficient.

[0095]The compensation upper limit input electric power calculation unit
11 calculates input electric power Plim such that electric power of
quadrature baseband signals I'' and Q'' after distortion compensation
operation becomes nearly equal to upper limit electric power after
compensation Pmax set by the limit value setting unit 10 in advance.
Upper limit electric power after compensation Pmax is, for example,
smallest input level with which a power amplifier becomes saturated. This
processing is, if amplitude compensation coefficient K for input electric
power Plim is α, processing to calculate Plim for which the formula

Pmax=α×Plim

[0096]holds. When a signal with input electric power exceeding input
electric power Plim is inputted to a power amplifier after distortion
compensation operation was performed, clipping distortion occurs in a
saturation region of the power amplifier. Therefore, perfect distortion
compensation cannot be performed. Accordingly, by employing input
electric power Plim as a value of the distortion compensable upper limit
of input electric power to the power amplifier, occurrence of the
clipping distortion can be prevented.

[0097]By the processing mentioned above, upper limit input electric power
Plim which gives upper limit electric power Pmax of quadrature baseband
signals I'' and Q'' after distortion compensation operation is obtained
according to the characteristics of a power amplifier. Also, when an
input signal is limited to no more than upper limit input electric power
Plim, clipping distortion of a power amplifier can be suppressed.

[0098]Accordingly, in the input electric power limit value decision unit
9, by using distortion compensable upper limit input electric power Plim
obtained by the compensation upper limit input electric power calculation
unit 11, and following the conditions below, electric power limit value
Pth which gives the amplitude limitation maximum value of a quadrature
baseband signal is decided.

[0102]The details and effects of the operation in each case mentioned
above are as follows. In case of i), because distortion compensable upper
limit input electric power Plim and electric power limit value Pth are
equal, it is judged that control is done as expected, and no change of
electric power limit value Pth is performed.

[0103]In case of ii), distortion compensable upper limit input electric
power Plim is smaller than electric power limit value Pth. This means
that characteristics of a power amplifier fluctuated and that input level
which gives saturated output level of a power amplifier, that is,
distortion compensable upper limit input electric power Plim declined. In
other words, there is a possibility that linearity of the power amplifier
is degraded. Therefore, it is necessary to lower electric power limit
value Pth to distortion compensable upper limit input electric power
Plim. That is, input amplitude maximum needs to be made small. By making
electric power limit value Pth small, it is possible to prevent large
signal amplitude from being inputted to the power amplifier, and to
suppress clipping distortion.

[0104]In case of iii), distortion compensable upper limit input electric
power Plim is larger than electric power limit value Pth. This means the
characteristics of power amplifier fluctuated, and input level which
gives saturated output level of a power amplifier, that is, distortion
compensable upper limit input electric power Plim has risen, is meant. In
other words, there is a possibility that linearity of the power amplifier
is improved. Therefore, it is possible to raise electric power limit
value Pth to distortion compensable upper limit input electric power
Plim. That is, input amplitude can be made large. By making electric
power limit value Pth large, it is possible, while suppressing
degradation of EVM (Error Vector Magnitude), to utilize characteristics
of a power amplifier to its maximum.

[0105]The input electric power limit value decision unit 9 outputs
electric power limit value Pth which was obtained as above to the
coefficient calculation unit 2. At this time, by obtaining several
electric power limit value Pth, their average may be taken and outputted
to the coefficient calculation unit 2. That is, the input electric power
limit value decision unit 9 may calculate reasonable number of electric
power limit value Pth chronologically, store them, take average Pth (ave)
of multiple calculated Pth, and output it to the coefficient calculation
unit 2.

[0106]As described above, in a distortion compensation circuit of the
third exemplary embodiment of the present invention, determination is
made whether to perform amplitude limitation based on electric power
limit value Pth. Power limit value Pth is decided according to the
characteristics of a power amplifier. Accordingly, amplitude limitation
of a quadrature baseband transmission signal can be performed adaptively,
and distortion compensation effects can be improved.

[0107]Also, using together a technology which applies limitation to signal
amplitude after distortion compensation or to a distortion compensation
coefficient itself, for example, the technology described in patent
document 1, is unnecessary. Accordingly, it becomes possible to calculate
a highly precise distortion compensation coefficient.

The Fourth Exemplary Embodiment

[0108]Next, a distortion compensation circuit of the fourth exemplary
embodiment of the present invention will be described with reference to
drawings. According to the third embodiment of the present invention,
judgment of whether to perform amplitude limitation is made based on
electric power limit value Pth. According to the fourth exemplary
embodiment of the present invention, judgment of whether to perform
amplitude limitation is made based on an amplitude limit value. FIG. 4 is
a block diagram showing a structure of a distortion compensation circuit
of the fourth exemplary embodiment of the present invention. For those
components in FIG. 4 equal to those in FIG. 3, an identical code is
attached.

[0110]The amplitude computation unit 101 computes amplitude V of
quadrature baseband signals I and Q and outputs it to coefficient
calculation unit 102. Amplitude V is proportional to a square root of
electric power P. Accordingly, amplitude V here is obtained by ignoring a
unit, and using the formula

V=(I2+Q2)1/2.

[0111]The coefficient calculation unit 102 compares V from the amplitude
computation unit 101 and amplitude limit value Vth given from the input
amplitude limit value decision unit 109. In case amplitude V is no more
than amplitude limit value Vth, the coefficient calculation unit 102
decides not to perform amplitude limitation and outputs 1 as a multiplier
coefficient in order not to limit amplitude of quadrature baseband
signals I and Q. On the other hand, in case amplitude V is larger than
amplitude limit value Vth, the coefficient calculation unit 102 decides
to perform amplitude limitation and outputs Vth/V as a multiplier
coefficient. Further, a multiplier coefficient when amplitude limitation
is performed is not limited to this value, and any value such that
amplitude V of quadrature baseband signals I and Q becomes not more than
amplitude limit value Vth will do.

[0116]The input amplitude limit value decision unit 109 outputs decided
amplitude limit value Vth to the coefficient calculation unit 102. Also,
by obtaining several amplitude limit value Vth, their average may be
taken and outputted to the coefficient calculation unit 102. That is, the
input amplitude limit value decision unit 109 may calculate reasonable
number of amplitude limit value Vth chronologically, store them, take the
average Vth (ave) of the multiple calculated Vth, and output it to the
coefficient calculation unit 102.

[0117]Upper limit amplitude Vmax fixed in advance is set in the limit
value setting unit 110. It is desirable that a value of upper limit
amplitude Vmax is set to input amplitude which makes output of a power
amplifier saturated, that is, a square root of saturated input level in
order to utilize the performance of the power amplifier to its maximum.
Further, upper limit amplitude Vmax is not necessarily set to a square
root of saturated input level, and may be set to a value smaller than the
square root.

[0118]The compensation upper limit input amplitude calculation unit 111
refers to the distortion compensation coefficient data memory 7, and
calculates input amplitude Vlim such that amplitude (square root of
electric power of a quadrature baseband signal) of a quadrature baseband
signal after distortion compensation operation becomes upper limit
amplitude Vmax which is set by the limit value setting unit 110. In other
words, if amplitude compensation coefficient K for input electric power
expressed as a square of input amplitude Vlim is α, then, the
compensation upper limit input amplitude calculation unit 111 calculates
amplitude Vlim such that the formula

[0120]Operation of each of other components of FIG. 4 is similar to each
corresponding component of FIG. 3. Further, in FIG. 4, amplitude
proportional to a square root of electric power P' may be used instead of
electric power P' of quadrature baseband signals I' and Q'. In this case,
the distortion compensation coefficient data memory 7 stores distortion
compensation coefficients K and θ by correlating them to amplitude.

[0121]As described above, in a distortion compensation circuit of the
fourth exemplary embodiment of the present invention, determination is
made whether to perform amplitude limitation based on amplitude limit
value Vth. Amplitude limit value Vth is decided according to the
characteristics of a power amplifier. Accordingly, amplitude limitation
of a quadrature baseband transmission signal can be performed adaptively,
and distortion compensation effects can be improved.

[0122]Also, using together a technology which applies limitation to signal
amplitude after distortion compensation or to a distortion compensation
coefficient itself, for example, the technology described in patent
document 1, is unnecessary. Accordingly, it becomes possible to calculate
a highly precise distortion compensation coefficient.

[0123]As above, first to fourth exemplary embodiments of the present
invention were explained in detail using drawings. An actual structure of
the present invention is not limited to the first exemplary embodiment,
and an exemplary embodiment to which design change was made to the extent
that does not deviate from the scope of the present invention is also
included in the present invention. Also, first to fourth exemplary
embodiments can be combined each other.

Fifth and Sixth Exemplary Embodiment

[0124]A distortion compensation circuit of the fifth exemplary embodiment
of the present invention is a distortion compensation circuit for
compensating distortion which occurs in an amplifier for amplifying a
quadrature baseband transmission signal and includes the following
components. That is, it includes: a coefficient calculation means which
determines whether to perform amplitude limitation of a quadrature
baseband transmission signal by comparing electric power of a quadrature
baseband transmission signal and an inputted limit value, and when
determined to perform amplitude limitation, outputs a coefficient for
making electric power of a quadrature baseband transmission signal not
more than an inputted limit value as a multiplier coefficient; an
amplitude limitation means which performs amplitude limitation by
multiplying the multiplier coefficient to a quadrature baseband
transmission signal; a distortion compensation coefficient calculation
means which calculates a phase compensation coefficient by performing an
operation to obtain a difference between an output signal of the
amplitude limitation means and a feedback signal which fed back part of
output signals of an amplifier; a storage means which stores the phase
compensation coefficient by correlating it with electric power or
amplitude of an output signal of the amplitude limitation means; a
distortion compensation operation means which performs distortion
compensation to an output signal of the amplitude limitation means based
on the phase compensation coefficient, and a limit value calculation
means which, by referring to stored contents of the storage means,
calculates electric power of an output signal of the amplitude limitation
means before distortion compensation operation when electric power of an
output signal of the amplitude limitation means after distortion
compensation operation by the distortion compensation operation means
becomes nearly equal to a predetermined limit value, as an inputted limit
value.

[0125]A distortion compensation method of the sixth exemplary embodiment
of the present invention is a distortion compensation method of a
distortion compensation circuit for compensating distortion which occurs
in an amplifier for amplifying a quadrature baseband transmission signal
and includes the following steps. That is, it includes: a coefficient
calculation step which determines whether to perform amplitude limitation
of a quadrature baseband transmission signal by comparing electric power
of a quadrature baseband transmission signal and an inputted limit value,
and when determined to perform amplitude limitation, outputs a
coefficient for making electric power of a quadrature baseband
transmission signal not more than an inputted limit value as a multiplier
coefficient; an amplitude limitation step which performs amplitude
limitation by multiplying the multiplier coefficient to a quadrature
baseband transmission signal; a distortion compensation coefficient
calculation step which calculates a phase compensation coefficient by
performing an operation to obtain a difference between a quadrature
baseband transmission signal after amplitude limitation and a feedback
signal which fed back part of output signals of an amplifier; a
distortion compensation operation step which performs distortion
compensation to a quadrature baseband transmission signal after amplitude
limitation based on the phase compensation coefficient, and a limit value
calculation step which, by referring to stored contents of a storage
means which stores the phase compensation coefficient by correlating it
to electric power or amplitude of a quadrature baseband transmission
signal after amplitude limitation, calculates electric power of the
signal concerned before distortion compensation operation when electric
power of a quadrature baseband transmission signal after distortion
compensation operation by distortion compensation operation means becomes
nearly equal to a predetermined limit value, as an inputted limit value.

[0126]According to the fifth and the sixth exemplary embodiment of the
present invention, effects are obtained to improve distortion
compensation effect by performing amplitude limitation of a quadrature
baseband transmission signal adaptively.

[0127]Although the present invention has been described with reference to
an exemplary embodiment above, the present invention is not limited to
the above-mentioned exemplary embodiment. Various changes which a person
skilled in the art can understand can be performed in the structure of
the present invention or details to the extent that does not deviate from
the scope of the present invention.

[0128]This application claims priority based on Japanese Patent
Application No. 2008-138817 filed on May 28, 2008 and the disclosure
thereof is incorporated herein in its entirety.

Patent applications by Junya Ashita, Tokyo JP

Patent applications in class Plural channels for transmission of a single pulse train

Patent applications in all subclasses Plural channels for transmission of a single pulse train